Buddleia (Buddleja davidii) is one of those rogue plants sometimes regarded as a giant weed for its ability to establish itself all over the place and spread rapidly, including cracks and crevices up the sides of buildings. It's a plant of forest clearings and riverbanks in its native China; also the fissures of limestone outcrops hence its ability to take hold in the lime mortar between bricks.
 If it's not causing a problem there's a lot to be said for Buddleia, flowering as it does later in the summer than many other plants in the UK. Waste ground, railway embankments and other neglected areas are all painted lilac, purple and white by Buddleia at this time of year (not to mention people's gardens).
 It's also known as the Butterfly Bush because no other plant attracts so many species of butterfly, and bumblebees like it too. The invasive qualities of B. davidii have a huge benefit for sustaining pollinators, particularly in urban areas.